sync_file_range() permits fine control when synchronizing the open
file referred to by the file descriptor fd with disk.
offset is the starting byte of the file range to be synchronized.
nbytes specifies the length of the range to be synchronized, in
bytes; if nbytes is zero, then all bytes from offset through to the
end of file are synchronized. Synchronization is in units of the
system page size: offset is rounded down to a page boundary;
(offset+nbytes-1) is rounded up to a page boundary.
The flags bit-mask argument can include any of the following values:
SYNC_FILE_RANGE_WAIT_BEFORE
Wait upon write-out of all pages in the specified range that
have already been submitted to the device driver for write-out
before performing any write.
SYNC_FILE_RANGE_WRITE
Initiate write-out of all dirty pages in the specified range
which are not presently submitted write-out. Note that even
this may block if you attempt to write more than request queue
size.
SYNC_FILE_RANGE_WAIT_AFTER
Wait upon write-out of all pages in the range after performing
any write.
Specifying flags as 0 is permitted, as a no-op.
Warning
This system call is extremely dangerous and should not be used in
portable programs. None of these operations writes out the file's
metadata. Therefore, unless the application is strictly performing
overwrites of already-instantiated disk blocks, there are no
guarantees that the data will be available after a crash. There is
no user interface to know if a write is purely an overwrite. On
filesystems using copy-on-write semantics (e.g., btrfs) an overwrite
of existing allocated blocks is impossible. When writing into
preallocated space, many filesystems also require calls into the
block allocator, which this system call does not sync out to disk.
This system call does not flush disk write caches and thus does not
provide any data integrity on systems with volatile disk write
caches.
Some detailsSYNC_FILE_RANGE_WAIT_BEFORE and SYNC_FILE_RANGE_WAIT_AFTER will
detect any I/O errors or ENOSPC conditions and will return these to
the caller.
Useful combinations of the flags bits are:
SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE
Ensures that all pages in the specified range which were dirty
when sync_file_range() was called are placed under write-out.
This is a start-write-for-data-integrity operation.
SYNC_FILE_RANGE_WRITE
Start write-out of all dirty pages in the specified range
which are not presently under write-out. This is an
asynchronous flush-to-disk operation. This is not suitable
for data integrity operations.
SYNC_FILE_RANGE_WAIT_BEFORE (or SYNC_FILE_RANGE_WAIT_AFTER)
Wait for completion of write-out of all pages in the specified
range. This can be used after an earlier
SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE operation
to wait for completion of that operation, and obtain its
result.
SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE |SYNC_FILE_RANGE_WAIT_AFTER
This is a write-for-data-integrity operation that will ensure
that all pages in the specified range which were dirty when
sync_file_range() was called are committed to disk.

EBADF fd is not a valid file descriptor.
EINVAL flags specifies an invalid bit; or offset or nbytes is
invalid.
EIO I/O error.
ENOMEM Out of memory.
ENOSPC Out of disk space.
ESPIPE fd refers to something other than a regular file, a block
device, a directory, or a symbolic link.

sync_file_range2()
Some architectures (e.g., PowerPC, ARM) need 64-bit arguments to be
aligned in a suitable pair of registers. On such architectures, the
call signature of sync_file_range() shown in the SYNOPSIS would force
a register to be wasted as padding between the fd and offset
arguments. (See syscall(2) for details.) Therefore, these
architectures define a different system call that orders the
arguments suitably:
int sync_file_range2(int fd, unsigned int flags,off64_t offset, off64_t nbytes);
The behavior of this system call is otherwise exactly the same as
sync_file_range().
A system call with this signature first appeared on the ARM
architecture in Linux 2.6.20, with the name arm_sync_file_range().
It was renamed in Linux 2.6.22, when the analogous system call was
added for PowerPC. On architectures where glibc support is provided,
glibc transparently wraps sync_file_range2() under the name
sync_file_range().

This page is part of release 3.82 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
http://www.kernel.org/doc/man-pages/.
Linux 2014-08-19 SYNC_FILE_RANGE(2)